1. Technical Field
The present invention relates to method for managing internal equipment in a reactor pressure vessel during operation of a nuclear power plant.
2. Background Art
Internal equipments grouped close together such as instrumentation nozzles mounted to the bottom portion of a reactor pressure vessel, penetrating a bottom head of the reactor pressure vessel, are joined to the bottom head by welding. Methods for carrying out preventive maintenance or repair of these welds have been conventionally performed in an underwater environment, so the applicable methods have been limited to methods performable in water, methods in which a region around a target is defined and locally made into an air environment, or methods in which cooling water in the reactor pressure vessel is drained from the reactor pressure vessel to make the entire vessel an air environment.
One method for preventive maintenance is, in terms of a material factor that is one of the causes for stress corrosion cracking, a method for reforming surface of structure member by weld-overlay using a corrosion-resistant welding material. One method for repair is repair welding after a crack is removed from structure member (for example, the reactor pressure vessel) composing a reactor by grinding. Thus, it is preferable that welding can be performed to the structure member in both the preventive maintenance and repair, and in order to perform welding in a reactor pressure vessel boundary, all procedures including pre-weld polishing, welding, after-weld polishing, weld size measurement, and penetrant testing (PT) are required to be performed in the given environment, all of which have been previously performed in air.
A plurality of prior arts disclosed a method applicable in water, such as using an underwater welding apparatus and a polishing/grinding apparatus, can be found. For example, Japanese Patent Laid-open No. Hei 9 (1997)-1347 discloses an underwater TIG welding apparatus that dome-forming gas is injected toward the tip of a TIG welding torch to eliminate water around a welding surface. Furthermore, in an underwater polishing apparatus disclosed in Japanese Patent Laid-open No. 2005-297090, a apparatus having a disc grinder for grinding/polishing and a hood cover is proposed; and there is a method individually combining the disc grinder and the hood cover.
As a method of limiting a target region and locally making an air environment surrounding the target region, a method using a cover apparatus for repairing a housing penetrating a pressure vessel (Japanese Patent Laid-open No. Hei 7 (1995)-318681), for example, has been proposed. In this method, a tubular sealing pipe surrounding a housing penetrating a reactor pressure vessel is provided to form a partial air space around the single housing penetrating the reactor pressure vessel.
As an example of a method for draining cooling water from a reactor pressure vessel to make entire inner region of the reactor pressure vessel into an air environment, a method for replacing an incore monitor housing and a apparatus used therefor has been proposed by Japanese Patent Laid-open No. 2001-108784. In this replacement method, since in a boiling water reactor (BWR), a shielding effect of cooling water cannot be obtained when the cooling water is drained from the reactor pressure vessel, causing radiation equivalent rate on an operation floor to rise, a thick shielding body is installed to a flange surface of the reactor pressure vessel and covers the reactor pressure vessel to drain the cooling water and thus replacement of an incore monitor housing can be performed in an air environment in the reactor pressure vessel.
In Japanese Patent Laid-open No. Hei 5 (1993)-312992, as in Japanese Patent Laid-open No. 2001-108784, a method has been proposed in which, after cooling water is drained below an operation target in a reactor pressure vessel, a chamber body is set and fixed using a flange surface of a top end of a core shroud disposed in the reactor pressure vessel and a lug, then a sealing plug is installed to a nozzle portion located in the upper portion of a jet pump disposed inside the reactor pressure vessel to prevent a flow of the cooling water into the lower portion of the reactor pressure vessel, so that various heads can be inserted in the lower portion of the reactor pressure vessel for preventive maintenance and repair work.